The present invention relates to a process for the production of sulphuric acid from sulphuric dioxide containing feed gas. More particularly, the invention is directed towards production of sulphuric acid with a feed gas having varying SO2 concentrations by adjusting SO2 concentration in the gas passed to a sulphuric acid plant through absorbing at least a part of SO2 in the feed gas in the aqueous solution or desorbing at least a part of SO2 from the aqueous solution.
Industrial off-gases containing SO2 are typically treated in a sulphuric acid plant, where the SO2 is oxidised to SO3 and recovered as concentrated sulphuric acid. An example is the large-scale production of sulphuric acid from SO2 containing off-gases from roasting and smelting of non-ferrous metal ores containing sulphides of e.g. Cu, Mo, Zn, Pb and Ni. Depending on the roasting and smelting process, these off-gases typically contain 1–40 mole % SO2, 2–10 mole % water vapour, oxygen, carbon dioxide, nitrogen and a small amount of SO3 in the form of sulphuric acid mist. However, the SO2 concentration may vary considerably with time, especially if the upstream process is a batch operation, in which case the SO2 concentration may fluctuate from zero in some periods of time to, say, 10–20 mole % in other time periods. Large variations in SO2 concentration or gas flow will lead to great disturbances and control difficulties in a downstream sulphuric acid plant and furthermore require a plant designed for a much larger capacity than the average SO2 flow.
From U.S. Pat. No. 3,764,665 a process is known for removing sulphur oxides from gas mixtures with a solid acceptor for sulphur oxides, wherein the solid acceptor is regenerated with a steam-diluted reducing gas and the regenerator off-gas is fed to a Claus sulphur recovery process. The regeneration off-gas is cooled and contacted with an SO2-selective liquid such as water thereby absorbing the SO2 from the gas. The SO2-rich liquid is passed to a buffer zone of such a volume that the SO2 concentration in the liquid in the buffer zone remains substantially constant. A stream of SO2-rich liquid is withdrawn from the buffer zone and passed to a stripping zone, where the SO2-rich liquid is heated to a temperature at which the SO2 is expelled. By operating in this manner, fluctuations in the SO2 concentration of the regeneration off-gas are levelled out and a relatively concentrated SO2 stream is obtained at a substantially constant rate.
A similar process for sulphuric acid production from process gas containing SO2 obtained from discontinuous processes is known from EP patent No. 091,938. According to this process, part of the SO2 in the feed gas is separated to form a liquid phase during periods in which process gas containing SO2 is generated in an upstream process, while the remaining SO2 in the gas is passed to a sulphuric acid plant. During those periods in which no process gases are produced, the stored SO2 containing liquid is passed to a stripping step where SO2 is stripped from the liquid with the aid of a carrier gas and passed to the sulphuric acid plant.
Disadvantages of the processes of U.S. Pat. No. 3,764,665 and EP patent No. 091,938 are the complexity of the process, which requires separate absorption and stripping steps.